Certain types of ink jet printers typically create a printed image by ejecting ink through orifices contained in an ink jet print head onto an image receiving medium. The image receiving medium may take the form of a sheet of media or an intermediate transfer drum that transfers the image to a sheet of media. Repeated printing builds up contaminates, such as unused ink and debris from the print medium, in the orifices and on the orifice plate. To ensure a high quality printed image, the print head must be periodically cleaned of this contamination to provide an unhindered ink trajectory from the orifices.
A typical conventional cleaning sequence, or print head maintenance cycle, entails drawing ink and debris from the orifices onto the orifice plate and then wiping the orifice plate. One example of a conventional cleaning sequence is disclosed in U.S. Pat. No. 5,570,117 entitled PRINT HEAD MAINTENANCE METHOD AND APPARATUS WITH RETRACTABLE WIPER, and assigned to the assignee of the present application. This cleaning sequence involves placing a sealable cap over the print head orifice plate and providing a single, limited duration vacuum within the cap by the activation of a separate vacuum reservoir, pump and activation valve. The vacuum draws trapped gasses and ink from the print head through the print head jet orifices.
In one prior art example of practicing the above cleaning sequence, the peak vacuum pressure utilized is in the range of -10 psig or greater, with "greater" corresponding to a greater negative valve, such as a -12 psig. The duration of the vacuum is approximately 0.375 seconds. A decaying vacuum profile follows the peak vacuum pressure for approximately 6-10 seconds until ambient pressure is achieved. A wiping action across the orifice plate is then used to remove excess ink from the surface of the orifice plate. In this example, the purge efficiency of the cleaning sequence is approximately 75%, with "purge efficiency" being defined as the percentage of time that the cleaning sequence succeeds in clearing all of the print head jets for proper operation.
The prior art print head maintenance cycle described above has several disadvantages. Utilizing a single vacuum pulse requires a higher peak vacuum pressure to achieve an acceptable purge performance. This higher peak vacuum pressure can cause cavitation in the print head and lead to the introduction of air bubbles or gasses pulled from ink solution. The process of forming the seal between the purge cap and the orifice plate can also force air into the orifices. It would also be desirable to improve the purging efficiency and reduce the amount of ink utilized in the maintenance cycle. Accordingly, a need exists for an improved print head maintenance method that cleans a print head while overcoming the disadvantages of the prior art.